Method of making an oven controlled crystal oscillator the frequency of which remains ultrastable under temperature variations

ABSTRACT

The present invention provides for methods of increasing the frequency vs. temperature (&#34;f vs. T&#34;) stability of Oven Controlled Crystal Oscillators to levels which are superior to atomic frequency standards and 100 to 10,000 times higher than that currently available from the best crystal oscillators. This method encompasses the steps of making an SC-cut quartz resonator with upper and lower turnover temperatures at or near the resonator&#39;s inflection temperature, inserting the resonator into a high-stability oscillator circuit, placing the circuit into a high-stability, high thermal gain oven and adjusting the oven temperature to a set-point at or near one of the resonator&#39;s turnover temperatures. A preferred embodiment of the method of the present invention is also disclosed which comprises forming an SC-cut quartz resonator with upper and lower turnover temperatures within 10 K of the resonator&#39;s inflection temperature, inserting the resonator into a dual-mode high-stability oscillator circuit which is placed in an oven having a thermal gain exceeding 5,000 and temperature fluctuations smaller than 50 mK and utilizing a thermometric beat frequency of said resonator to adjust the oven temperature to a set-point within 100 mK of the lower turnover temperature.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used and licensed byor for the Government of the United States of America without thepayment to us of any royalties thereon.

FIELD OF THE INVENTION

The present invention relates generally to the field of frequencycontrol and timing technology and more particularly to methods ofattaining higher frequency stability with respect to temperaturevariations of piezoelectric devices, particularly Oven ControlledCrystal Oscillators.

BACKGROUND OF THE INVENTION Description of the Prior Art

Piezoelectric crystal devices are used primarily for precise frequencycontrol and timing, with quartz being the piezoelectric crystal used inmost applications. A quartz crystal acts as a stable mechanicalresonator, which by its piezoelectric characteristics and high Q valuecan determine the frequency generated in an oscillator circuit. Of thefour basic types of crystal oscillators known, the Oven ControlledCrystal Oscillator ("OCXO") is of particular interest in the presentinvention.

Until now, the atomic frequency standard exhibited superior stability tothe crystal oscillator in the areas of frequency vs. temperaturecharacteristics and aging. Atomic frequency standards have been wellknown for many years. They provide the highest standard of accuracyheretofore available. While atomic frequency standards embodied in theatomic clock have been commercially available for some time and haveundergone many improvements, they still suffer from a number ofdisadvantages, including being cumbersome due to their large size, andhaving high power requirements, limited temperature range and,particularly in the case of beam type standards, a limited life.

The method of the present invention provides frequency vs. temperature("f vs. T") stabilities for an OCXO which are from 100 to 10,000 timesgreater than presently available. The f vs. T stability of an OCXOdepends on a number of factors, including (1) the static and dynamic fvs. T characteristics of the resonator in the sustaining circuit; (2)the difference between the oven's set-point and the point where thestatic f vs. T characteristic has a zero slope; (3) the oven'stemperature excursions from the set-point or oven "cycling" range, and(4) the rate of change of temperature during the oven's temperatureexcursions.

Those concerned with the development of crystal oscillators, as well asatomic frequency standards, have long recognized the need for higher fvs. T stability. The present invention provides for a method ofincreasing the f vs. T stabilities of an OCXO which meets this long-feltneed and suffers from none of the disadvantages of previously availablecrystal oscillators with a lesser f vs. T stability such astemperature-compensated crystal oscillators, prior art OCXO's andrubidium frequency standards.

In general, the method of the present invention encompasses the steps ofmaking an SC-cut quartz resonator with turnover temperatures at or nearthe inflection temperature, inserting the resonator into ahigh-stability oscillator circuit, placing the resonator in ahigh-stability, high thermal gain oven and adjusting the oventemperature to a set-point at or near one of the turnover temperaturesto provide for higher f vs. T stability in an OCXO.

The temperature stable OCXO produced in accordance with method of thepresent invention would be extremely useful in applications whererecalibration capability, such as that attained with the aid of a GlobalPositioning System ("GPS") is frequently, but not continuouslyavailable. An example is a mobile communications operation where theuser returns to a base station which has a recalibration facility wherethe OCXO can be automatically recalibrated while batteries are beingrecharged. Such a temperature-stable OCXO along with a recalibrationsystem would be more advantageous in certain situations than having anOCXO continuously locked onto GPS.

The prior art in this field may be found in the following references:

J. A. Kusters, "The SC-Cut Crystal-An Overview," Proceedings of IEEEUltrasonics Symposium, 1981, pp. 402-409;

E. P. EerNisse and J. A. Kusters, "Orientation Dependence of `True`SC-Cuts," Proceedings 44th Annual Symposium on Frequency Control, 1990,pp. 185-192;

J. R. Vig and F. L. Walls, "Fundamental Limits on the FrequencyInstabilities of Quartz Crystal Oscillators," Proceedings IEEE FrequencyControl Symposium, 1994, published Oct. 1994;

C. A. Adams, D. C. Bradley & J. A. Kusters, "X-Ray Technology-A Review,"Proceedings 41st Annual Symposium on Frequency Control, 1987, pp.249-257;

J. R. Vig, W. Washington & R. L. Filler, "Adjusting the Frequency vs.Temperature Characteristics of SC-Cut Resonators by Contouring,"Proceedings of the 35th Annual Symposium on Frequency Control, 1981, pp.104-109;

R. L. Filler and J. R Vig, "Resonators for the Microcomputer-CompensatedCrystal Oscillator," Proceedings of the 43rd Annual Symposium onFrequency Control, 1989, pp. 8-15;

F. L. Walls, "Analysis of High Performance Compensated ThermalEnclosures," Proceedings 41st Annual Symposium on Frequency Control,1987, pp. 439-443;

J. A. Kusters, et. al., "A No-Drift and Less than 1×10⁻¹³ Long TermStability Quartz Oscillator Using a GPS S/A Filter," Proceedings 1994IEEE International Frequency Control Symposium, 1994, published Oct.1994; and

J. R. Vig, U.S. Pat. Ser. No. 4,375,604 entitled "Method of AngleCorrecting Doubly Rotated Crystal Resonators," Mar. 1, 1983.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an OCXO exhibiting an f vs.T stability superior to that of atomic frequency standards.

It is another object of this invention to provide an OCXO exhibiting fvs. T stabilities 100 to 10,000 times higher than that currentlyavailable from the best crystal oscillators.

To attain these and other objects, the present invention contemplates amethod of increasing the f vs. T stability of an OCXO comprising thesteps of making an SC-cut quartz resonator with turnover temperatures ator near the resonator's inflection temperature, inserting said resonatorinto a high-stability oscillator circuit, placing said circuit into ahigh-stability, high thermal gain oven and adjusting the oventemperature to a setpoint at or near one of the turnover temperatures ofsaid resonator. A preferred embodiment of the method of the presentinvention is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and details of the present invention will become apparentin light of the Detailed Description of the Invention and theaccompanying figure.

FIG. 1 is a chart depicting the f vs. T characteristic of an SC-cutresonator. The f vs. T characteristic of a resonator is a graph showinghow the resonator's frequency varies with the resonator's temperature.

"FIG. 2 is a schematic drawing showing the oven controlled crystaloscillator, the quartz resonator, the oscillator circuit and ovenutilized in the method of applicants' invention."

Additionally, three tables are included within this disclosure.

DETAILED DESCRIPTION OF THE INVENTION

As described in the Background of the Invention, those concerned withthe development of crystal oscillators, as well as atomic frequencystandards, have long recognized the need for higher f vs. T stability.The present invention provides for a method of increasing the f vs. Tstabilities of an OCXO which meets this long-felt need and suffers fromnone of the disadvantages of previously available crystal oscillatorswith a lesser f vs. T stability such as temperature-compensated crystaloscillators, prior art OCXO's and rubidium frequency standards.

The present invention addresses the long-felt need for greater f vs. Tstability, without any of the disadvantages of prior devices such ascumbersome size, high power requirements, a limited temperature rangeand the limited life of beam type standards. The present inventionprovides a method of increasing the f vs. T stability of an OCXOcomprising the steps of forming an SC-cut quartz resonator with an upperand a lower turnover temperature, each being at or near said resonator'sinflection temperature, inserting said resonator into a high-stabilityoscillator circuit which is placed in a high-stability, high thermalgain oven and adjusting the oven temperature to a set-point at or nearone of said turnover temperatures of the resonator.

The preferred embodiment of the method of the present inventioncomprises forming an SC-cut quartz resonator having an upper and a lowerturnover temperature, each being within 10 Kelvins ("K") of theinflection temperature of said resonator, inserting said resonator intoa dual-mode high-stability oscillator circuit which is placed in an ovenhaving a thermal gain exceeding 5,000 and temperature fluctuationssmaller than 50 millikelvins ("mK") and utilizing a thermometric beatfrequency of said resonator to adjust the oven temperature to aset-point within 100 mK of said lower turnover temperature of theresonator.

The f vs. T stability of an OCXO depends upon four primary factors,specifically: (1) both the static and dynamic f vs. T characteristics ofthe resonator in the sustaining circuit; (2) the difference between theoven's set-point and the point where the static f vs. T characteristichas a zero slope, meaning the oven's offset from the turnover point; (3)the oven's temperature excursions from the set-point, also known as theoven "cycling" range, and (4) the rate of change of temperature duringthe oven's temperature excursions. The dynamic f vs. T stability candominate the static f vs. T stability when the resonator is not of thetype that is thermal transient compensated, that is when the resonatoris not an SC-cut crystal.

In order to attain maximum f vs. T stability, forming or selecting athermal transient compensated resonator is one of the steps of themethod of the present invention. While many SC-cut resonators exhibit afinite thermal transient effect, when operating in the vicinity of a"true" thermal compensated cut, small changes in the angles of the cutscan produce either positive or negative thermal transient effects. A"true" thermal compensated cut is one where the thermal transient effectis exactly 0 at one set of angles of cut in the vicinity of θ=34° andθ=22°. The exact values of the "true" SC-cut angles vary with resonatorconfiguration. The OCXO's dynamic f vs. T performance can also beinfluenced by factors such as transient effects in oven control andsustaining circuit components, therefore these factors must also beminimized during oven design.

Research demonstrates that with proper care, extremely high f vs. Tstabilities are achievable and results similar to those shown in TABLE Ibelow illustrate significant possibilities in this area. TABLE I depictsfrequency offsets due to thermal cycling as a function of the offset ofthe oven set-point from the resonator turnover point, which is:

    T.sub.i -T.sub.TP =10K

The static f vs. T characteristic assumed in TABLE I was based upon anSC-cut resonator having a turnover point 10K from the inflection point.It should be noted, that the assumed f vs. T characteristic in TABLE Iis not the optimum f vs. T characteristic that can be achieved. Thesmaller the T_(i) -T_(Tp) resonator turnover point, the higher is theOCXO's f vs. T stability capability.

                  TABLE I                                                         ______________________________________                                        for T.sub.i -                                                                          Oven Cycling Range (mK)                                              T.sub.TP = 10K                                                                         100       10        1       0.1                                      ______________________________________                                        Oven  1000   4 × 10.sup.-10                                                                    4 × 10.sup.-11                                                                  4 × 10.sup.-12                                                                  4 × 10.sup.-13                   Offset                                                                              100    6 × 10.sup.-11                                                                    4 × 10.sup.-12                                                                  4 × 10.sup.-13                                                                  4 × 10.sup.-14                   (mK)  10     2 × 10.sup.-11                                                                    6 × 10.sup.-13                                                                  4 × 10.sup.-14                                                                  4 × 10.sup.-15                         1      2 × 10.sup.-11                                                                    2 × 10.sup.-13                                                                  6 × 10.sup.-15                                                                  4 × 10.sup.-16                   ______________________________________                                    

Referring now to FIG. 1, this chart depicts the f vs. T of an SC-cutresonator with a zero f vs. T slope at the inflection temperature. FIG.1 depicts the f vs. T characteristic for the optimum resonator, which isa resonator where the turnover temperatures coincide with the inflectiontemperature. While FIG. 1 depicts f vs. T characteristics of the optimumresonator, such a resonator is difficult to fabricate. A cubic f vs. Tcharacteristic was assumed in curve-fitting the FIG. 1 data andcalculating the TABLE I figures. For small temperature excursions near aturnover temperature, the cubic approximation provides adequateaccuracy. With careful X-ray orientation, angle correction,recontouring, design and fabrication, achieving such an f vs. Tcharacteristic is possible. It is noted that the f vs. T characteristicdepicted in FIG. 1 is from a resonator produced by recontouring, asdescribed more fully in the above-cited J. R. Vig, W. Washington & R. L.Filler reference, and the above-cited patent reference of J. R. Vig, oneof the co-inventors herein. If a recontoured resonator is utilized, thefigures shown in TABLE I become much smaller, as is demonstrated inTABLE II below.

TABLE II below depicts frequency offsets due to thermal cycling, as afunction of the offset of the oven set-point from the resonator turnoverpoint, which is T_(i) -T_(Tp) =0K, and similar to both FIG. 1 and TABLEI, a cubic f vs. T was assumed for calculating the TABLE II figures.

                  TABLE II                                                        ______________________________________                                        for T.sub.i -                                                                          Oven Cycling Range (mK)                                              T.sub.TP = 0K                                                                          100       10        1       0.1                                      ______________________________________                                        Oven  1000   2 × 10.sup.-11                                                                    2 × 10.sup.-12                                                                  2 × 10.sup.-13                                                                  2 × 10.sup.-14                   Offset                                                                              100    4 × 10.sup.-13                                                                    2 × 10.sup.-14                                                                  2 × 10.sup.-15                                                                  2 × 10.sup.-16                   (mK)  10     8 × 10.sup.-14                                                                    4 × 10.sup.-16                                                                  2 × 10.sup.-17                                                                  2 × 10.sup.-18                         1      6 × 10.sup.-14                                                                    8 × 10.sup.-17                                                                  4 × 10.sup.-19                                                                  2 × 10.sup.-20                   ______________________________________                                    

The degree of accuracy of oven temperature setting to the optimumset-point depends primarily on the hysteresis of the resonator and ofthe thermometer. Secondary factors are the noise of the resonator,thermometer and amplifier. If, for example, the normalized beatfrequency of between three times the fundamental mode frequency minusthe third overtone frequency is used as the thermometer, then the slopeof the thermometric beat frequency is about 10⁻⁴ /K, and the hysteresisof this type of SC-cut resonator, over a much wider temperature than isnecessary for setting the oven to turnover, is about 10⁻⁸. Thereforebased on TABLE II, in principle, the temperature can be set to withinabout 10⁻⁴ K of the optimum set-point.

Oven stability depends on both the temperature changes in the oven'senvironment as well as the thermal design of the oven. Excellent ovenstability can be obtained, even with a single oven, if the oven isproperly designed. A thermal gain of 10⁵ can be obtained with a singleoven. This means that, for example, if the environment changes by 100K,the inside temperature of the oven changes by only 1 mK. According toTABLE II, if the oven is stable to 1 mK, then as long as the oven offsetis approximately 0.5K, the OCXO's instability due to temperaturefluctuations will be less than its short term instability, σ₆₅ (τ)floor, i.e., less than 10⁻¹³. Therefore, the f vs. T stability can besuperior to those of even the best atomic frequency standards.

FIG. 2 illustrates the main features of subjects invention, in which 1is the oven controlled crystal oscillator, 2 is the quartz resonator, 3is the oscillator circuit, 4 is the oven, and 5 is thermal insulationwhich thermally isolates the oven from the outside world.

TABLE III below shows typical f vs. T stabilities of the major types ofcommercially available atomic frequency standards. It is clear fromTABLES II and III that the f vs. T stabilities comparable to those ofthe atomic standards can be achieved with a suitably made OCXO.

                  TABLE III                                                       ______________________________________                                        Atomic standard                                                                              f vs. T   Temperature                                          type           stability range (°C.)                                   ______________________________________                                        Small rubidium 3 × 10.sup.-10                                                                    -55 to +65                                           Militarized cesium                                                                           2 × 10.sup.-11                                                                    -28 to +65                                           Commercial cesium                                                                            1 × 10.sup.-13                                                                     0 to +50                                            ______________________________________                                    

If such all OCXO also has an aging rate of, for example, 2×10⁻¹¹ perday, which the best available OCXO's can achieve after extendedoperations, then the OCXO can maintain a frequency accuracy equal orbetter than the accuracy of a miniature rubidium standard, for periodsof up to about two weeks. There are tradeoffs such as retrace effectsand longer stabilization time. Retrace effects, parts in 10¹⁰ even inthe best OCXO, would require the OCXO to remain continuously powered orrecalibrated after turn-on in order to maintain this state of highaccuracy. Also, OCXO's do require a longer stabilization time in orderto achieve parts in 10¹¹ per day aging rates. On the other hand, OCXO'shave the advantages of better reliability and short term stability,lower cost, longer life, smaller size and power requirements, as well asthe ability to operate over a wider temperature range than atomicstandards can.

The first embodiment of the method of the present invention comprisesthe steps of forming an SC-cut quartz resonator having an upper and alower turnover temperature at or near an inflection temperature of saidresonator, inserting said resonator into a high-stability oscillatorcircuit, heating said circuit in a high-stability., high thermal gainoven and adjusting the oven temperature to a set-point at or near one ofthe turnover temperatures to provide for higher f vs. T stability in anOCXO.

The preferred embodiment of the method of the present inventioncomprises the steps of forming an SC-cut quartz resonator having anupper and a lower turnover temperature within 10K of an inflectiontemperature of said resonator, inserting said resonator into a dual-modehigh-stability oscillator circuit, heating said circuit in an ovenhaving a thermal gain exceeding 5,000 and temperature fluctuationslesser than 50 mK, and utilizing a thermometric beat frequency of saidresonator to adjust the temperature of said oven to a set-point within100 mK of the lower turnover temperature of said resonator.

Variations of the first and preferred embodiments include the followingadditional steps of the method of the present invention. For example,forming said resonator to have a turnover temperature less than 1K ofthe inflection temperature, and selecting the set-point of the oven morethan 100 mK from the lower turnover temperature. Moreover, the method ofthe present invention also encompasses advantageously selecting aset-point near an upper turnover temperature, rather than the lowertemperature. Further, the method of the present invention alsoencompasses a step of selecting a resonator having the necessaryattributes, rather than forming such a resonator.

Accordingly, having shown and described what are at present consideredto be the preferred and several embodiments of this invention, it shouldbe understood that the same have been shown by way of illustration andnot limitation. It should be understood, of course, that the foregoingdisclosure relates only to a small number of preferred embodiments andthat numerous modifications and alterations may be made therein withoutdeparting from the spirit and the scope of the invention as set forth inthis disclosure and the appended claims. All modifications, alterationsand changes coming within the spirit and scope of the invention arehereby meant to be included.

What we claim is:
 1. A method for increasing frequency vs. temperaturestability of an Oven Controlled Crystal Oscillator, comprising the stepsof:forming a quartz resonator with an upper turnover temperature and alower turnover temperature near an inflection temperature of saidresonator, said resonator being nearly thermal transient compensated;inserting said resonator into a high-stability oscillator circuit;placing said circuit in a high-stability, high thermal gain oven; andadjusting said oven to a set-point near said upper turnover temperature.2. The method for increasing frequency vs. temperature stability of anOven Controlled Crystal Oscillator, as recited in claim 1, wherein saidresonator is thermal transient compensated.
 3. The method for increasingfrequency vs. temperature stability of an Oven Controlled CrystalOscillator, as recited in claim 2, wherein said resonator is SC-cut,having angles of cut in the vicinity of θ=34° and φ=22°.
 4. The methodfor increasing frequency vs. temperature stability of an Oven ControlledCrystal Oscillator, as recited in claim 1, wherein said resonator isformed having said upper turnover temperature and said lower turnovertemperature at said inflection temperature of the resonator.
 5. Themethod for increasing frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 4, wherein saidset-point is adjusted to said upper turnover temperature.
 6. The methodfor increasing the frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 1, wherein saidset-point is adjusted near said lower turnover temperature.
 7. Themethod for increasing the frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 6, wherein saidset-point is adjusted to said lower turnover temperature.
 8. The methodfor increasing the frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 3, further comprisingthe steps of:forming said resonator with said upper turnover temperatureand said lower turnover temperature being within 1K of said inflectiontemperature; and adjusting said set-point to be within 100 mK of saidlower turnover temperature.
 9. The method for increasing frequency vs.temperature stability of an Oven Controlled Crystal Oscillator, asrecited in claim 1, wherein the high-stability, high thermal gain ovenpossesses a thermal gain exceeding 5,000.
 10. A method for increasingfrequency vs. temperature stability of an Oven Controlled CrystalOscillator, comprising the steps of:forming a quartz resonator with anupper turnover temperature and a lower turnover temperature within 10Kof an inflection temperature of said resonator, said resonator beingnearly thermal transient compensated; inserting said resonator into adual-mode, high-stability oscillator circuit; placing said circuit in ahigh-stability, high thermal gain oven having a thermal gain above 5,000and a plurality of temperature fluctuations lower than 50 mK; andadjusting said oven to a set-point within 100 mK of said lower turnovertemperature.
 11. The method for increasing frequency vs. temperaturestability of an Oven Controlled Crystal Oscillator, as recited in claim10, wherein said resonator is thermal transient compensated.
 12. Themethod for increasing frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 11, wherein saidresonator is SC-cut, having angles of cut in the vicinity of θ=34° andφ=22°.
 13. The method for increasing frequency vs. temperature stabilityof an Oven Controlled Crystal Oscillator, as recited in claim 12,further comprising the step of utilizing a thermometric beat frequencyof said resonator when adjusting said oven.
 14. The method forincreasing frequency vs. temperature stability of an Oven ControlledCrystal Oscillator, as recited in claim 10, wherein said resonator isformed having said upper turnover temperature and said lower turnovertemperature at said inflection temperature of the resonator.
 15. Themethod for increasing the frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 14, wherein saidset-point is adjusted to said lower turnover temperature.
 16. The methodfor increasing the frequency vs. temperature stability of an OvenControlled Crystal Oscillator, as recited in claim 13, furthercomprising the steps of:forming said resonator with said upper turnovertemperature and said lower turnover temperature being within 1K of saidinflection temperature; and adjusting said set-point to be within 100 mKof said lower turnover temperature.